Update Image service property keys doc
Update Image service property keys doc

Doc update for change Ie8227fececa40e502aaa39d77de2a1cd0cd72682

Change-Id: I4b35d0ae768b331945cd014c004cef16ed2155b3
Closes-bug: #1694441

openstack/python-glanceclient,openstack/python-glanceclient